This clinical study consists of two phases. In Phase I, participants who satisfy inclusion criteria will receive ketamine at variable doses (0.1mg/kg-0.5mg/kg) or a placebo (saline, or 0.01mg/kg midazolam) once a week over up to 6 weeks. If participants qualify for Phase II, they will receive repeated sessions of ketamine at variable doses over three weeks. During both phases, mood, psychiatric, and neuropsychological outcomes will be measured.